An automatic mask alignment technique using moiré interference

This paper deals with an automatic and precise alignment technique for x‐ray lithographic masks, using two pairs of moire gratings, with their respective moire signals 180° out of phase with each other. The moire signals are detected in the 0th order beams, and the difference between these two signals are used to control the alignment of an x–y stage through a stepping motor. Experiments were carried out by using a transmission grating pitch of 200 μm, a displacement corresponding to one step of the stepping motor as 28 nm, a pulse frequency of 15 Hz and a response time of 0.13 s. With these parameters a precision better than 120 nm was obtained. Experiments were carried out by using a reflection grating of 25 μm pitch. The minimum displacement corresponding to one step of the stepping motor was 14 nm. The resolution of the A/D converters was 12 bits and the response time was 1.2 s. With these parameters a control reproducibility better than 32 nm was obtained. Improvement in the control precision by an o...